There is a trend within the field of portable communicating devices, and especially within the field of cellular phones to have the antenna in-built in the phone itself. At the same time there is often a need for several frequency bands for such phones. The reasons for providing different frequency bands are several. First of all some networks provide several frequency bands in order to better distribute traffic in the network. GSM does for instance provide two separate bands that can be used. There are also different types of networks in different countries that use different frequency bands.
It is not a simple task to provide an antenna structure that can be used with good efficiency in several such bands, especially if the antenna is to be based on a PIFA antenna and provided in a stick type phone.
Various methods have been developed to achieve the additional bandwidth such as using parasitic elements that can be end coupled or side coupled, capacitively fed structures, ultra-high structures and electrically matched structures.
Parasitic elements can be useful in achieving additional bandwidth. They may be end coupled or coupled with parallel elements. When end coupling is used, it is necessary to have the space for two elements, which essentially doubles the required antenna volume. It has also been noted, particularly at low-band frequencies, that there is a tendency when parasitic elements are used for a low-gain region to occur in the frequencies between the resonant frequencies of the two resonators.
Several capacitively fed structures have been proposed. These structures can, in general, achieve the required bandwidth, but generally with a reduction in efficiency. When the bandwidth for instance at the low GSM band is doubled, a reduction in gain of 2–3 dB has generally been observed.
Ultra-high structures (13–15 mm in height) have shown promise in some Japanese products. The main problem with these devices is that they cause the overall size of the product to grow to what is often an unacceptable level.
Electrically matched structures have promise in theory. Currently there are a number of ways of accomplishing this, each with some drawbacks. PIN Diodes work well when properly biased. However, the necessary bias current to limit harmonic generation can lead to reduced standby and talk times. MEMS technology is in theory the best solution to eliminate harmonic and current consumption issues. However, currently devices are generally unavailable and highly sensitive to ESD. GaAs switches currently show the most promise. They have insertion loss of around 0.3 to 0.5 dB. The use of such structures furthermore adds to the cost of the product and require additional power, which it is necessary to keep as low as possible in a portable communication device. Since switching between some bands is only required very seldom, like for instance when travelling from Europe to USA, the use of a fast switching, which electrically matched structures provide, is not an important factor.